Honey bees are the most important commercial pollinator world-wide for agricultural production and food security. Agriculture depends on animal pollination for over 100 different crops that provide >30% of the human diet and also supply fiber, fuel, and drugs. [3, 4] Honey bees are responsible for 80% of managed insect pollination and their economic impact on U.S. food crops in the year 2000 has been estimated to be $14.6 billion. [3] However, honey bees and other insect pollinators suffer from a combination of factors, including habitat loss, pesticide exposure, pathogens, and stress due to active management. [15, 16] Recently, honey bee health has been marked with sharp declines and losses of colonies, with reports of “colony collapse disorder.” [17] The failure to identify a single factor as the cause of colony collapse disorder indicates that the global honey bee health crisis is complex and heterogeneous: Multiple stressors may act synergistically to lead to symptomatic health declines and colony failure [18] and the causes may vary in time and between locations. [19]
While no single factor has been identified as the cause of colony collapse disorder, the introduction and spread of parasites and associated pathogens and pesticide exposure have a central role in the health decline of honey bees. [17, 20-29] Disease control in the honey bee is crucial because it remains the most important commercial pollinator world-wide and its pollination services are irreplaceable in many agricultural systems. Moreover, honey bees are significant pollinators in natural ecosystems and are potential vectors of diseases that threaten native pollinator communities. [6]
The ectoparasitic mite Varroa destructor in particular is a threat to honey bee health and apiculture today. [30, 31, 33, 41, 42] Varroa enter honey bee colonies to complete its reproductive cycle on developing honey bee brood. [30, 31] Mature, fertilized Varroa females enter a brood cell to lay eggs, and both the Varroa female and its offspring feed on the brood. [35] Varroa causes physical and physiological damage when feeding on the brood. Varroa is a vector of viruses and has been associated with viral amplification and honey bee disease susceptibility. Miticides used to control Varroa infestations are problematic because of toxicity to honey bees, beekeepers, and crops; general ecosystem pollution; and resistance development in Varroa. 
Varroa is conventionally treated with synthetic acaricides, most notably coumaphos, tau-fluvalinate, flumethrin, and amitraz. [31] These substances are toxic and persistent and may accumulate in the hive [64], consequently harming honey bee health. [65, 66] For example, miticides fluvalinate and coumaphos, have been found to have lethal and sublethal effects on honey bee queens, workers, and drones. [2, 31, 64-66, 132, 154, 162, 177, 181] Moreover, resistance build-up in resident Varroa populations decreases the efficacy of chemical control. [31, 72, 178] Synergistic effects of fluvalinate and coumaphos have been measured, where the toxicity of each chemical is significantly increased in bees previously exposed to the other. [172] Furthermore, immunosuppression caused by chemical exposure makes honey bees more susceptible to parasites like Varroa, as well as to the pathogens they vector. [64, 70, 181, 184] In addition to affecting honey bee health, miticides compromise beekeeper health [158], enter bee products including those consumed by humans [64, 157, 174, 185] and contribute to general ecosystem pollution.
Other control strategies such as physical mite removal and use of organic acids and essential oils have been proposed as alternatives, but have many limitations that compromise efficacy, such as the labor-intensive application, temperature-sensitivity, and potential side-effects on honey bees. [74, 75, 171] Despite substantial evidence of the need, no adequate solution for control of Varroa has been developed. [75]
Traditional efforts to keep honey bees healthy have focused on management techniques and treatments (primarily chemotherapies). These treatments may lead to a loss of honey, are expensive and labor-intensive, and pose human health risks. [62, 63] Moreover, their long-term efficacy is questionable: None of the honey bee diseases have been eradicated due to past management. On the contrary, a steady emergence of novel pests and pathogens can be observed. [19]
Beekeepers have selectively bred honey bees for hygienic behavior as an alternative. [75, 87, 92, 155, 183] The mechanism for hygienic behavior is not completely understood, but the trait can be measured, for instance, by frequency of certain behaviors in honey bees. Minnesota Hygienic (HYG) is a breed of honey bees based on the high frequency of honey bee removal of freeze-killed brood. [83] A circle of brood is frozen with liquid nitrogen and percent removal of the killed brood is recorded, thus assessing the effectiveness of the general detection and removal of dead brood. [83] However, the olfactory trigger for hygienic removal of mite-infested and other live brood may be significantly lower than that of dead brood. [10, 100, 112, 156, 175, 183] HYG breeding relies on olfactory triggers of dead brood and results in bred honey bees that may lack sufficient sensitivity to living diseased brood. [94, 100] Another example is Varroa Sensitive Hygienic (VSH), which is a breed of honey bees based on measured changes in mite reproduction. [85] VSH breeding is based on a more narrowly defined goal but hygienic behavior may just be one mechanism for these bees to suppress mite reproduction. It has been unclear whether VSH bees are truly distinct or whether suppression of mite reproduction is due to the interruption of the mite reproductive cycle by hygienic removal of infested brood. [31, 84, 85, 170, 196] While the HYG and VSH hygienic honey bee colonies exhibit reduced mite and disease loads compared to unselected hives [9, 84, 167, 182], the selective breeding programs are not widely adopted by beekeepers due to lack of specificity, difficulty and expense of current selection assays. [92-95, 99, 183, 197] Also, hygienic lines do not yet serve as complete alternatives to chemical Varroa control [75], as chemical treatments are sometimes required to control severe mite infestations in hygienic hives. [170, 182]
Moreover, despite the presence of natural honey bee social immune mechanisms like hygienic behavior, honey bee health is currently being severely threatened. Following global pollinator population trends [2, 189], managed honey bee colonies in the United States have declined steadily for over six decades, from 5.9 million colonies in 1947 to 2.4 million colonies in 2005 [190, 191]. Total annual colony losses in the United States have exceeded 33% in four of the last five years, exceeding 45% between April 2012 and April 2013. [192, 193, 194, 195] Honey bee losses are largely attributed to introduction and spread of new parasites and associated pathogens, and to lethal and sublethal effects of agrochemical exposure. [2, 47, 195]
The long-term sustainability of apiculture depends on the balance between the benefit of honey bee keeping to the individual and the costs of honey bee management and losses. There remains a need for improving selection of honey bees for hygienic behavior and use of hygienic behavior by honey bees to prevent or treat a diseased honey bee colony to ensure adequate supplies of managed pollinators for agriculture.